During the Miocene, several reefs formed in the Beikang Basin, South China Sea, which may be potential targets for hydrocarbon exploration. This is due to the environment that developed as a result of the collision, splitting, and splicing of the Nansha Block, which was influenced by the Neogene expansion of the area. However, studies on the types, distribution, controlling factors, and evolution stages of these reefs are scarce. In this study, we used high-resolution seismic data and extensive well-drilling records to gain insights into the evolution of reefs in this particular area. Six distinct types of reefs, namely, the point reef, the platform-edge reef, the block reef, the bedded reef, the pinnacle reef, and the atoll reef, were identified based on our data. These reefs underwent four stages of development. During the initial stage, a few small-sized point reefs emerged in the basin and experienced significant growth during the early Middle Miocene. In the flourishing stage, the reefs predominantly thrived around the Central Uplift and Eastern Uplift areas. In the recession stage, the reefs began to deteriorate during the late Middle Miocene period as a result of the rapid increase in relative sea level caused by tectonic subsidence. In the submerged stage, since the Late Miocene, as the relative sea level continued to rise steadily over time, many reefs that had previously flourished surrounding the Central Uplift and Eastern Uplift areas became submerged underwater, with only a handful of atoll reefs surviving near islands located on the Eastern Uplift. This study indicated the presence of a significant number of well-preserved reefs in the Beikang Basin that have experienced minimal subsequent diagenesis and therefore exhibit high potential as reservoirs for oil and gas exploration.

为明确西沙海域晚中新世深水沉积体发育特征及影响因素,根据二维和三维地震资料,结合区域构造背景,建立深水水道发育模式.结果表明:西沙周缘中新世发育两类不同物源类型的深水沉积体,一类是以越东水系陆源碎屑供源的深水水道,另一类是以西沙台地碳酸盐岩碎屑供源的深水水道.陆源碎屑供源的深水水道,主要受古地形和海平面升降的影响,水道的迁移性强,水道间存在相互切割的特征;碳酸盐岩碎屑供源的深水水道,主要受古地形的影响,以垂向加积为主,水道的迁移性弱.该结果对西沙周缘油气勘探中深水水道的识别与储层预测具有指导意义.

西沙海域夹持于南海西北次海盆和西南次海盆之间,构造演化过程与南海的扩张和南海西部的走滑作用关系密切.基于覆盖西沙海域的区域地震资料开展了构造-地层解释、盆地结构特征分析和区域构造演化制图,整体上将西沙海域划分出3种类型盆地,即高角度断层控制的盆地、低角度拆离断层控制的盆地和走滑盆地.结合地壳厚度变化和伸展薄化程度,突出断层的构造样式,将西沙海域划分为北部拆离断层构造发育区、东南部拆离断层构造发育区、西部走滑断层发育区和中部高角度断层发育区,进而明确了西沙海域盆地的基本构造格局.同时,以关键构造界面为主线,强调了不同类型断层在岩石圈地壳减薄过程中的作用,阐明了西沙海域盆地的差异构造演化过程.

西沙海域位于南海西北部陆坡区,受陆缘张裂和走滑的双重控制,经历了断陷和拗陷2个构造演化阶段,受越东、红河、海南岛和西沙等4个物源区的共同影响,充填了陆相—过渡相—海相地层,发育火山、气烟囱、块体搬运和水道等多种地质体,孕育了多种类型的海底地貌.基于丰富的二维和三维地震资料,应用三维可视化等技术,完整直观地展示了西沙海域海底地貌.将位于大陆坡的研究区海底地貌细分为海底平原、海底斜坡和海槽等3个三级海底地貌,进一步细分为深海平原、深海水道、深海蜂窝状、深海海山、深海月牙状、深海似圆状、深海蝌蚪状、深海海槽和深海条带状共9个四级地貌.结合地质条件分析了各种地貌成因及展布的主控因素:深海平原、深海水道和深海海槽等3种海底地貌主要受控于断陷期的构造格局和晚期的沉积充填;深海蜂窝状地貌与块体搬运体系伴生;深海海山为火山活动成因,底流有改造作用;深海月牙状地貌是底流侵蚀的产物;深海似圆状、深海蝌蚪状和深海条带状地貌是气体逸散造成的不同形状的海底麻坑.这项成果有助于西沙海域构造特征、深水沉积类型与分布、地质体展布及海流等方面的深化研究.

ABSTRACTThe incision history of the Three Rivers (Salween, Mekong, and Yangtze) region in the Southeast Tibetan Plateau has been linked to both tectonic and climatic controls. In this study, we report new apatite (U‐Th)/He and fission‐track thermochronology data from the >6,000‐m‐high Kawagebo massif, which forms the edge of the high plateau on the western flank of the steepened knickzone reach of the middle Mekong River valley. Thermal‐history modeling of a thermochronological age‐elevation profile shows rapid cooling since ~1.5 Ma and suggests a mean Quaternary exhumation rate of >1 km/Myr at the valley bottom. The amount of Quaternary exhumation is too high to be caused by fluvial incision alone and requires additional tectonic uplift. Comparing our data from the western flank of the Mekong River valley with published data from the eastern flank shows differential exhumation across the valley in the late Miocene, with the western flank undergoing more exhumation, but relatively uniform exhumation in the Quaternary. We relate rapid exhumation since the late Miocene on the western flank of the Mekong valley and the high topography of the Kawagebo massif to localized tectonic uplift associated with a restraining (left stepping) overstep between the still‐active right‐lateral Parlung and Zhongdian strike‐slip faults. The pattern of river steepness index across the knickzone also indicates that it results from locally focused uplift. Our results demonstrate the importance of detailed thermochronologic studies in this very active region to constrain the complex multiphase tectonic history before invoking any potential climatic forcing of river incision.

通过对南海周边新生代板块活动的分析,认为南海构造演化由太平洋板块、欧亚板块和印度板块等3大板块间的相互作用所控制,并可分为断陷阶段(E)、断拗转换阶段(N11—N12)和区域性沉降阶段(N13—现今).将南海已发现油气藏归纳为构造圈闭型、岩性圈闭型和复合型3大类,进一步将构造圈闭型分为背斜型、断背斜型、断块(鼻)型3个亚类,将岩性圈闭型分为生物礁滩型和深水水道型2个亚类,将复合型分为构造-岩性型和基岩潜山型2个亚类,并对典型油气藏特征进行了分析.总结了南海油气藏在平面和纵向上的分布特征,认为南海演化的不同阶段形成了不同的圈闭类型和不同的储层,从而控制了3大类油气藏的分布.平面上,构造型油气藏主要分布在南海的南北两侧靠近大陆边缘地区,以油为主;生物礁滩型油气藏分布在远陆缘区,以气为主;而西部的走滑断陷盆地除了发育构造型油气藏,还发育复合型油气藏,西北部以构造-岩性型为主,西南部基岩潜山型占优势.纵向上,构造型、生物礁滩型油气藏发育均有北早南晚的特点;西部走滑盆地复合型油气藏发育层位北部比南部更新;深水水道型油气藏集中在上中新统—上新统,发育层位最新.

We employ recently updated seismic reflection data to investigate spatial-temporal difference of Cenozoic faults systems and its dynamic mechanism of the Pearl River Mouth Basin and Qiongdongnan Basin in the Northern of South China Sea.It shows geometry, kinematics of extensional faults systems in the Paleogene rift zone of the Pearl River Mouth basin and Qiongdongnan basin change significantly along strike, which is composed of NE-and EW-trending normal basement faults.The con-cealed regional and local NW-trending faults with associated deformation zone constitute the transfer zones between the extensional faults systems.The "non-detachment" extensional fault system consists of NE-and NEE-trending normal faults to the west of NW-trending Yunkai and Songtao-Songnan transfer zones, and the detachment fault system is composed of NE-and EW-trending normal/strile-slip faults to the east.Paleogene rifting experienced three distinguished stages.In the early period (Wenchang Formation), simple (half) grabens in western basins was controlled by planar normal faults with relatively small extension, and composite (half) grabens in eastern basins are dominated by listric normal faults with large extension, where normal faults converge and terminate at detachment fault within mid-crust.In the middle period (Enping/Yacheng Formation), faults in the west are characterized by inherited activity, whereas NWW-and EW-trending faults in the east of Pearl River Mouth Basin form deep detachment system by cutting antecedent mid-crust detachment fault, with inversion along the Yunkai transfer zone.In the late stage (Zhuhai/Lingshui Formation), the activities of faults decline in the west, whereas NWW-and EW-trending faults form deep detachment system in the east of the Qiongdongnan Basin, with inversion and strike-slip structures along the Songtao-Songnan transfer zone.The influencing factors of differential fault systems and tectonic evolution include pre-existing basement fabrics, rheology of crust and lithosphere, and extension amount.The detachment systems are associated with the NWW-trending throughgoing crustal faults within basement weak belts and present local crustal thinning zone, the geodynamic mechanisms for differential extension on both sides of the transfer zones result from the strike-slip movement along the NWW-trending pre-existing faults induced by the opening of the South China Sea from east to west, and changes of the stress field and lithospheric thermal structure account for the depth variation of the detachment systems.

Using thermochronological data and thermokinematic modeling, we constrain the timing of late Miocene exhumation of the northern portion of the Gongga batholith, located in Southeast Tibet along the Xianshuihe Fault (XF). We show that rapid exhumation started in the north of the Gongga batholith at ∼9 Ma at a rate of ∼1.85 km/Myr and slowed down since ∼4 Ma. A magmatic pulse occurring during the early Pliocene (∼4 Ma) has overprinted the rapid Miocene exhumation phase in some parts of the batholith, which record mainly early Pliocene post-magmatic cooling. Slow exhumation since ∼4 Ma is consistent with the present-day lower relief observed in the centre of the batholith, which contrasts with the rugged high peaks located to the south. We propose that the northern segment of the XF, the Yalahe fault, which is not active at present, was active between 9 and 4 Ma, forming a restraining bend that focused exhumation south of it. Since ∼4 Ma, the Selaha and the Zheduotang faults form the present-day restraining bend south of which the highest part of the massif is located, including the Gongga Shan that rises more than 3000 m above the mean elevation of the plateau. In the north of the batholith, similar peaks have been removed since the Miocene by local relief reduction at high elevations. Considering that the onset of motion along the XF is contemporaneous with the onset of rapid exhumation recorded along the Kangding transect at ∼9 Ma and a total offset of ∼62 km documented for the XF, the average slip rate of the XF is ∼7 mm/yr since 9 Ma.

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (similar to 4200-4800m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16Ma, AHe ages from 2 to 6Ma, and one zircon fission track age is similar to 99Ma. These data imply a tectonic quiet period sustained since at least 100Ma with a slow denudation rate of similar to 0.03km/Ma, interrupted at 7 to 5Ma by exhumation at a rate between 0.59 and 0.99km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an hidden plate-tectonic model rather than with a lower channel flow model.

Xunhua-Hualong basin Cenozoic sedimentary strata, basement and surrounding mountains analysis of apatite fission track geochronology reveals the Northeast margin of Qinghai-Tibet plateau experienced three periods of uplift and exhumation events since Late Cretaceous: (1) Basin basement, Laji mountain and West Qinling orogenic belt apatite fission track ages generally record a Late Cretaceous-Eocene relatively rapid regional uplift and exhumation event. Northern margin of West Qinling rapidly uplift at 84 Ma, which was controlled by the uplift of the northward thrust; to the North Lamu Gorge in central of Xunhua-Hualong basin uplift at 69 Ma; further north rapid uplift stages along Laji mountain is mainly between 40-50 Ma, reflecting Paleocene-Eocene rapid uplift gradually extending from south to north. This tectonic uplift event resulted in Xunhua-Hualong basin and Linxia basin missing Paleogene Xining group sedimentary strata that deposits in northern Xining-Minhe basin. The first period of uplift and exhumation ended at about 31 Ma,when Xunhua-Hualong basin connected with eastern Linxia basin to a large unified foreland basin of West Qinling, both experience the same tectonic and sedimentary evolution history, therefore Cenozoic oldest Tala group sedimentary strata age in Xunhua-Hualong basin wont exceed the oldest sedimentary strata with Paleomagnetic age of 29 Ma in Linxia basin. (2) Late Oligocene about 26 Ma Laji mountain began bi-directionally thrust-uplift and may have continued to Early Miocene about 21 Ma, and uplifting made Xunhua-Hualong basin become piedmont extrusion foreland basin between Laji mountain and West Qinling, when Xunhua-Hualong basin began to deposit Tala group alluvial fan of thick coarse clastic rocks. By analyzing sedimentary facies, the ancient flow direction and gravel composition of Xianshuihe group and Linxia group sedimentary strata in Xunhua-Hualong basin, it is concluded that Laji mountains experienced the largest bi-directional thrust-uplift event in the Miocene about 8 Ma, which led directly to the event of Xunhua-Hualong piedmont extrusion foreland basin turning into mountain basin, which formed basic pattern of basin-mountain landform in northeastern margin of Qinghai-Tibet plateau nowadays.